California is the leading producer of raisins in the United States. About 97% of all California raisins are from the variety Thompson seedless, also called Sultana in Australia and Europe. There are three main methods by which raisins are produced in California. One method is to dip the grapes into a caustic soda and water solution followed by exposure to sulphur/CO.sub.2 and then dried in a dehydrator to create golden colored raisins. These raisins have a certain following and are distinguished by their golden color and their slightly sulfur taste. A second type of raisin made in California is by a hot water dipped process. In this process green grapes are dipped in a hot water solution which not only slightly splits the skins to promote subsequent drying but also causes the resulting raisin to have a darkish (brown) color and a taste vaguely reminiscent of chocolate.
By far the most prevalent type of raisins sold in the United States is the natural Thompson seedless (NTS) raisin. NTS raisins are dried in the sun. In this sun drying process, green grapes are picked when sufficiently ripe, typically about 21.degree. Brix, and placed on drying trays or strips of paper. The drying trays or paper strips are placed on dirt mounded or terraced between the rows of vines. It usually takes about 3 weeks to sun-dry raisins in the Central Valley of California. This produces the conventional dark-colored, NTS raisin in a simple and cost-effective manner.
One of the problems with dried-on-the-ground type raisins is the chance of rain during the drying season. If rained on, dried-on-the-ground raisins are susceptible to mold, infestation, rot and may be ruined. Also, especially under more humid conditions, these raisins might be attacked by certain molds or mildew which would ruin them for human consumption. Another problem with dried-on-the-ground raisins is the cost associated with picking the grapes, placing the trays on the dirt mounds formed between the rows, and gathering the raisins when dried.
Efforts to mechanically harvest green grapes and automatically deposit them onto strips of drying paper between the rows have been made. Because of the violence with which grapes are mechanically harvested (the grapes are beaten or shaken off their vines), a certain amount of damage and stickiness to the grapes occurs. While this is not a problem if the grapes are immediately crushed, such as occurs when making wine, leaving damaged grapes on drying trays on the ground for a number of weeks may create lower quality raisins and could result in the growth of molds and mildew as well as promote insect damage.
Other parts of the world, such as Australia, do not produce substantial amounts of natural raisins. Rather, the green grapes, when mature, are sprayed with an oil emulsion, typically 2% aqueous solution of methyl oleate and potassium carbonate. This creates tiny cracks in the skin to aid drying of the raisins and produce a lighter color raisin. The sprayed grapes are generally dried in large drying racks using chicken wire.
There is an Australian system for producing dried-on-the-vine raisins using what is called an Irymple trellis system. In that system divided canopies are created along the row of grape vines. To create divided canopies, the fruiting canes are supported on horizontal trellis systems along one side of the row while the replacement canes are supported on the other side of the row. The fruit develops from the fruiting canes and hangs freely below the leaf canopy beneath the fruiting canes. When the fruit is mature, the grape bunches are sprayed with an oil emulsion, such as methyl oleate, and the fruiting canes are cut to speed drying of the grapes. It has been found that cutting the fruiting canes at this time, so long as one limits the number of canes cut to no more than about 50% of the total canopy, does not injure the vine, which would reduce the next year's harvest.
The advantages of on-the-vine drying Thompson seedless grapes in the Central Valley area of California are compelling. However, the conventional wisdom on doing this is that it is not possible to vine dry Thompson seedless grapes in the Central Valley of California to produce raisins without the use of chemical treatment. For example, in a paper entitled "Vine-Drying of Thompson Seedless Grapes", Henry E. Studer, Harold P. Olmo, Transactions of the ASAE, Vol. 16, No. 5, pp. 944-952, 1973, the authors stated: "The feasibility of drying the fruit before harvest while it is hanging from the trellis wires on severed canes was casually examined in 1965 (Studer and Olmo, 1967). The conclusion was that weather conditions during the fall months in California's Central Valley are such that complete on-the-vine drying of the fruit into raisins was not practical." The conclusions of that article included the following: "The severed-cane technique permits partial drying of Thompson seedless grapes into raisins directly on the vine. . . . A final moisture content of 15% or less can be achieved by drying the fruit for approximately 12 hours in a tunnel dehydrator, or under deep bed drying conditions.
In an article entitled "Vine Drying Raisin Grapes In Coachella Valley", Henry E. Studer, California Grape Grower, March, 1988, pp. 16-20, the author stated "Attempts to use cane severing followed by drying on the vine to the Thompson seedless variety have been unsuccessful in the Central Valley of California." The article states that Zante currents, made from Black Corinth grapes in the Central Valley, have been successfully produced using on-the-vine drying. However, the Black Corinth grape variety has a very small berry which permits the on-the-vine drying. Some Thompson seedless vines are sprayed with an aqueous solution of methyl oleate and potassium carbonate to promote rapid drying of the fruit on the vine, the canes having been severed. This, of course, does not create natural raisins with their characteristic color and flavor. It also has the disadvantage of the cost associated with spraying and negative effects of the chemical spray on the vines. In summary, the author states that "In the San Joaquin Valley, acceptable varieties certainly do not exist, and natural drying on the vine is not presently a viable alternative to sun drying".
The consensus of opinion is that on-the-vine drying of Thompson seedless grapes in the Central Valley without the use of chemical sprays is not viable.